1. Field of the Invention
The invention relates to superplastically formed structures and, more particularly, to including a perforated sheet in a forming pack to be superplastically deformed into a structure for controlling laminar flow over an airplane.
2. Description of the Prior Art
The salutary aerodynamic characteristics of laminar flow have been obtained over aircraft surfaces where the flow would otherwise have become turbulent, by applying suction through a perforated aircraft skin. For optimal effect, aircraft require skin perforations that are extraordinarily small. For example, for a skin thickness of 0.040 of an inch, the perforations usually will have a diameter of less than 0.004 of an inch at the outer-facing surface of the skin, that is, the skin surface that is exposed to fluid flow.
It has also been found that the perforations are subject to clogging by airborne particles when the perforation has a tapered shape wherein its diameter at the exposed surface is larger than its diameter at the inner-facing or blind surface. The preferred shape is a taper wherein the diameter of the perforation at the blind surface is larger than its diameter at the exposed surface.
An electron beam or laser beam can drill perforations of the desired small diameter in skins composed of titanium alloys, the metals required by high speed airplanes because such alloys retain their strength at elevated temperatures. The perforations can be drilled through the skin either before the skin is incorporated into the aircraft structure, or after the structure has been manufactured. However, there are problems associated with each alternative.
Where the perforations are to be drilled through a skin that is already incorporated into the airplane structure, the electron or laser beam can drill perforations having a sufficiently small diameter at the exposed surface. However, the diameter of this perforation decreases as the depth of the perforation increases, such that the diameter of the perforation at the exposed surface is greater than the diameter at the blind surface. As previously noted, a perforation of this shape is susceptible to clogging.
An electron or laser beam can drill perforations having a diameter larger at the blind surface than at the exposed surface. The attendant problem is that such perforations have an outer surface diameter greater than the small diameter typically required for effective laminar flow control.
Further, when the perforations are made on a skin already attached to the aircraft structure, dust particles are created by the drilling process and fall into the structure. As the particles are extremely hot when they are formed, they oftimes adhere to the blind surface and are thus not easily removed because of the inaccessibility of the blind surface. Rather, the particles come loose when subjected to the vibration caused by flight, and can subsequently clog the perforations when the perforations are periodically subjected to reverse flow to clear them, or when hot air is forced through the perforations to de-ice the exposed surface of the skin.
In order to avoid the foregoing drawbacks, skins have been perforated to the required size and taper, and then fastened to the aircraft structure. The problem here lies in the means of fastening. Rivets must be anchored in a substructure situated beneath the skin. The substructure abuts the blind surface of the perforated skin and blocks the perforations. This reduces the area of the perforated exposed surface available to control laminar flow, and thus reduces the efficiency of the perforated skin in controlling laminar flow. Moreover, installing rivets is costly because it is labor intensive.
Adhesives also have been used to fasten the skin to the airplane structure. There are several problems with this approach. The strength of the adhesive bond is proportional to the abutting surface area of the two opposing surfaces being fastened to each other. As the blind surface of the perforated skin is fastened to an underlying solid substructure, the perforations are blocked across the area of attachment. The substantial surface area required by an adhesive thus directly reduces the area of the perforated exposed surface having unobstructed perforations and, concomitantly, reduces the efficiency of the perforated skin in controlling laminar flow. Furthermore, the strength of the adhesive weakens when repeatedly exposed to the extreme thermal cycles caused by typical flights.
Aircraft parts of exceptional strength and diverse configuration have been fabricated by superplastically deforming metallic sheets placed in abutment in forming packs. Though obviously desirable, the fabrication of a perforated skin for an airplane by superplastically deforming a perforated metallic sheet has not been achieved. The reason is that superplastic forming relies on the sustained application of a substantial pressure differential between the sheets of the forming pack. This pressure differential is created by the injection of a pressurized forming gas between the sheets. Leakage of the forming gas through the perforations of the sheet that is to form the perforated skin would prevent its superplastic deformation.